One type of imaging system employs photoconductive materials to absorb incident radiation representative of an image of an object. Suitable photoconductive materials will absorb the radiation and produce electron-hole pairs (charge carriers) which may be separated from each other by an electric field applied across the photoconductor, creating a latent image at the surface of the photoconductor (which is typically a thin planar layer). A narrow beam of scanning radiation substantially completes discharge of the photoconductor, by creating motion of a second set of charge carriers. The distribution of these second charge carriers in the plane of the photoconductor is affected by the distribution of the first charge carriers, i.e., by the latent image. The motion of the second charge carriers is detected and digitized in an appropriate circuit, and thus the latent image is captured in digital form.
In one specific embodiment, the photoconductor is part of a multilayer structure comprising two electrodes, between which are the photoconductive layer and an insulative layer. A voltage source maintains electric fields in the structure during exposures to the incident radiation and the scanning radiation (although not necessarily the same field strength is present during each exposure). An example of this type of system is taught in U.S. Pat. No. 4,176,275 (Korn et al). Application of the electric field across the photoconductive layer can be assisted by establishing a prior (reverse) field across the insulating layer, as taught in U.S. Pat. No. 4,539,591 (Zermeno et al).
A second and closely related approach, known as the air-gap photoinduced discharge (PID) method, employs air as the insulating layer, and requires that a uniform separation be maintained between the two electrodes, typically by high-precision mechanical or piezoelectric devices. A corona device such as a corotron charges the surface of the photoconductor prior to exposure to radiation, producing an electric field in the material. Thus, the incident radiation partially discharges the surface to produce a latent image, and the read-out signal is induced by the charge motion under the influence of the residual electric field in response to the scanning radiation. Such a system is described in Rowlands et al, Med. Phys. 18(3), May/June 1991 at 421-431.
Various methods for scanning the latent image exist. For example, the method of U.S. Pat. No. 4,961,209 (Rowlands et al) employs a transparent sensor electrode positioned over the photoconductive layer, and a pulsed laser which scans the photoconductive layer through the transparent sensor electrode. By moving the photoconductive layer and the transparent sensor relative to each other, so that the direction of relative motion is transverse to the direction in which the laser scans, a pixel-by-pixel discharge of the latent image charge is created.
Practical applications of these systems have encountered several problems.
First, fabrication of the imaging stack (i.e., the electrodes, insulator, photoconductive material, etc.) requires applying layers of material to each other, typically by constructing two sub-stacks, and then applying them to each other. These procedures can introduce non-uniformities into the thicknesses of the imaging stack.
Second, reflection and scattering of incident radiation can occur at the interfaces between layers, reducing image quality. This problem, and the attempted solutions to it, are compounded by the non-uniformities in thicknesses.
Third, discharge breakdown of the insulative material is possible, especially in the air-gap PID approach, leading to avalanche currents in the system.
Fourth, as the areal size of the imaging stack increases, a requirement of practical applications such as chest x-ray imaging, the capacitance created by the electrode plates increases, reducing the effectiveness of the system. One approach to this last problem is that of U.S. Pat. No. 4,857,723 (Modisette). This approach avoids, rather than solves, the capacitance issue, by employing many small detectors ganged together.
Many of these problems are addressed successfully by the system described in U.S. Pat. No. 5,268,569, Nelson et al, Imaging System Having Optimized Electrode Geometry and Processing. That system employs the electrode closest to the photoconductor as the detection electrode, segmenting this electrode into stripes, and optionally interconnecting several of these (e.g., every thirty-second stripe) to reduce the number of individual detection circuits.
However, in the Nelson et al system image resolution is effectively fixed by the physical size of each individual segmented stripe. Generally, a single stripe supports a single line of resolution of essentially the same width as the stripe width. Thus, the fixed stripe (and thus pixel) size means that higher resolution requires arrays of very narrow, unacceptably difficult-to-form segmented stripes, which then must be arranged over a larger area than is currently possible to accomplish at acceptable manufacturing yields. The problem is especially acute in medical diagnostic applications, which require pixel widths on the order of 20-200 microns, over increasingly larger image areas (e.g., as large as 14 inches (35.6 centimeters) by 17 inches (43.2 centimeters) for abdominal or chest imaging).
A second limitation of the Nelson et al. system is "charge spreading," a lateral motion of the charge carriers (due to space charge and in-transit charge-induced electric field distortion, especially at reduced readout field strengths) that can cause charges released over one stripe to be collected by a neighboring stripe. This causes degradation of image resolution (sharpness of the image) during the readout process. The extent of charge spreading can be approximately 10-20% of the photoconductor thickness, e.g., 50-100 microns in diagnostic x-ray systems.
A third limitation is significant inter-stripe (coplanar) capacitance. For narrow stripes, the coplanar capacitance seen by the detector circuit is large. Excessive capacitance at the input of a detector circuit can cause a significant amount of noise.
It is possible, although not as preferred, for a single narrow stripe of the Nelson et al system to support more than one pixel of the electrostatic image. This can be accomplished by using multiple scans (sub-scans) of an intensity-modulated laser spot having a size smaller than the width of a stripe, and scanning at a higher rate than would normally occur. Each scan involves modulation of the intensity of the smaller spot over a different sub-portion of the stripe, However, the need to modulate the intensity of the laser at precise times to place the pixels with high resolution and accuracy on the narrow stripes, introduces additional, and undesired, system hardware and software requirements.